A review of the present and future utilisation of FRP composites in the civil infrastructure with reference to their important in-service properties

Near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement is one of the latest and most promising strengthening techniques for reinforced concrete (RC) structures. Research on this topic started only a few years ago but has by now attracted worldwide attention. Issues raised by the use of NSM FRP reinforcement include the optimization of construction details, models for the bond behaviour between NSM FRP and concrete, reliable design methods for flexural and shear strengthening, and the maximization of the advantages of this technique. This paper provides a critical review of existing research in this area, identifies gaps of knowledge, and outlines directions for further research.

Near-surface mounted FRP reinforcement: An emerging technique for strengthening structures

This paper presents the results of a recent study conducted to refine the design-oriented stress–strain model originally proposed by Lam and Teng for fiber-reinforced polymer (FRP)-confined concrete under axial compression. More accurate expressions for the ultimate axial strain and the compressive strength are proposed for use in this model. These new expressions are based on results from recent tests conducted by the writers’ group under well-defined conditions and on results from a parametric study using an accurate analysis-oriented stress–strain model for FRP-confined concrete. They allow the effects of confinement stiffness and the jacket strain capacity to be separately reflected and accounts for the effect of confinement stiffness explicitly instead of having it reflected only through the confinement ratio. The new expressions can be easily incorporated into Lam and Teng’s model for more accurate predictions. Based on these new expressions, two modified versions of Lam and Teng’s model are present...

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Refinement of a design-oriented stress-strain model for FRP-confined concrete

In this paper, a stress–strain model for concrete confined by fiber reinforced polymer (FRP) composites is developed. The model is based on the results of a comprehensive experimental program including large-scale circular, square and rectangular short columns confined by carbon/epoxy and E-glass/epoxy jackets providing a wide range of confinement ratios. Ultimate stress, rupture strain, jacket parameters, and cross-sectional geometry were found to be significant factors affecting the stress–strain behavior of FRP-confined concrete. Such parameters were analyzed statistically based on the experimental data, and equations to theoretically predict these parameters are presented. Experimental results from this study were compared to the proposed semi-empirical model as well as others from the literature.

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Stress-strain model for concrete confined by FRP composites

The application of textile reinforced mortar (TRM) as a means of increasing the shear resistance of reinforced concrete members is investigated in this study. TRM may be considered as an alternative to fiber reinforced polymers (FRP), providing solutions to many of the problems associated with application of the latter without compromising much the performance of strengthened members. Based on the experimental response of reinforced concrete members strengthened in shear it is concluded that textile-mortar jacketing provides substantial gain in shear resistance; this gain is higher as the number of layers increases and, depending on the number of layers, is sufficient to transform shear-type failure to flexural failure. TRM jackets were provided in this study either by conventional wrapping of fabrics or by helically applied strips. Both systems resulted in excellent results in terms of increasing the shear resistance. However, compared with their resin-impregnated counterparts, mortar-impregnated textiles may result in reduced effectiveness. Modelling of reinforced concrete members strengthened in shear with TRM jackets instead of FRP ones is presented by the authors as a rather straightforward procedure by the proper introduction of experimentally derived jacket effectiveness coefficients. From the limited results obtained in this study it is believed that TRM jacketing is an extremely promising solution for increasing the shear resistance of reinforced concrete members.

Shear strengthening of reinforced concrete members with textile reinforced mortar (TRM) jackets

This paper reports on the key features of the “Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures” issued by the American Concrete Institute (ACI). Fiber reinforced polymers (FRP) systems have emerged as an alternative to traditional materials and techniques for the strengthening of existing concrete structures to resist higher design loads, correct deterioration-related damage, design or construction error, or increase ductility. Structural elements strengthened with externally bonded FRP systems include beams, slabs, columns, walls, joints/connections, chimneys and smokestacks, vaults, domes, tunnels, silos, pipes, and trusses. Externally bonded FRP systems have also been used to strengthen masonry, timber, steel, and cast-iron structures.

Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures (ACI 440.2R-02)

Reinforced concrete (RC) diaphragms play a fundamental role in seismic resistance of RC buildings. In addition to transferring the gravity loads to the beams and columns, diaphragms need to be sufficiently rigid with high shear capacity to resist and distribute the earthquake loads to the lateral force resisting system. Despite the large building inventory with under designed diaphragms to resist the shear loads from earthquakes, the number of studies investigating possible approaches to increase the in-plane shear capacity of diaphragms is limited. External equally distributed narrow strips of fiber-reinforced polymers (FRP) providing additional shear strength for RC diaphragms is proposed in this study. Five full scale RC diaphragms with overall dimensions of 4.5 m long by 1.5 m wide by 0.1 m thick were built and tested under cyclic shear loading. One out of the five specimens was tested as a reference with no strengthening, while the remaining four were strengthened with external glass or carbon fiber-reinforced polymer (GFRP or CFRP) composites. The test results showed that the FRP strengthening system effectively enhances the in-plane shear capacity of the RC diaphragms. Compared to the reference specimen, the FRP layout used for testing increased the shear capacity by up to 40%. This paper presents the test specimens, strengthening systems, and in-plane shear response of tested diaphragms in terms of failure modes, strength, and deformation. The experimental results were also compared to theoretical values determined according to the evaluation criteria in the International Code Council (ICC)-Evaluation Services (ES), Acceptance Criteria (AC) 125.

In-plane shear strengthening of reinforced concrete diaphragms using fiber reinforced polymer composites

This paper presents a deterministic optimization model that can be utilized for budget allocation to implement retrofitting measures for steel and concrete water pipelines. This model can be used by decision-makers to develop budget allocation strategies and to determine which strengthening and retrofit methods are the most practical and economical to be used considering a predefined set of limitations and criteria. The deterministic model determines the optimal retrofit measures based on minimizing the pipe retrofit costs, post-earthquake replacement costs, and especially large seismic losses. The main constraints considered for this model are mitigation budget constraint, mitigation strategy constraint, and risk of especially large loss constraint. A case study is performed for a pilot area per the return of certain earthquake to demonstrate the application and practical aspects of the model. Pipeline exposure database, built environment occupancy type, pipeline vulnerability functions, and regional seismic hazard characteristics are used to calculate a probabilistic seismic risk for the pilot area. The results provide the variation of retrofit costs for pipelines in the pilot area based on their geographical location and the selected strengthening measures. 

T. Alkhrdaji

Papers

Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures (ACI 440.2R-02)

In-plane shear strengthening of reinforced concrete diaphragms using fiber reinforced polymer composites

Deterministic Optimization Model for Budget Allocation to Seismic Risk Mitigation for Water Distribution System

Shear strengthening of reinforced concrete T-beams with anchored fiber-reinforced polymer composites